The displacement of a displacement pump stationary pressure chambers, such as a bent axis piston or gerotor pump, is commonly controlled through a control ring. The control ring is provided with openings in order to control the flow of pressure medium from the displacement pump. Typically, there are two openings, which are divided by an intermediate section, referred to as a land.
When pistons/pressure chambers pass over the land between the high pressure and low pressure opening of a control ring, the pressure is decreasing/increasing rapidly due to the land blocking the pressure medium. When the opening has passed over the land and reaches the opening in the control ring a pressure spike/pressure dip causing vibration and noise occurs due to the large pressure differences.
Pre-compression/pre-decompression and how it varies dependent of the position of the control ring. In a displacement pump with a control ring arranged for 100% displacement, the pressure rises slowly over the land. In a displacement pump with a control ring arranged for 0% displacement, the pressure rises very high over the land, causing high pressure differences between the outlet openings and the opening in the control ring to which it opens to. The standard solution of today is a compromise, in which the land is adapted such that it gives an ideal pre-compression in an intermediate position of the control ring. An obvious drawback to this solution is that in any other control position of the displacement pump, the pre-compression becomes to large/small, whereby the displacement pump still will have problems with pressure pulses and there from derived vibrations and disturbing noises. A displacement pump according to the prior art is disclosed in WO 12120094.